DOCSLIB.ORG

Pushdown Automata

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Pushdown Automata Pushdown Automata Announcements ● Problem Set 5 due this Friday at 12:50PM. ● Late day extension: Using a 72-hour late day now extends the due date to 12:50PM on Tuesday, February 19th. The Weak Pumping Lemma ● The Weak Pumping Lemma for Regular Languages states that For any regular language L, There exists a positive natural number n such that For any w ∈ L with |w| ≥ n, There exists strings x, y, z such that For any natural number i, w = xyz, w can be broken into three pieces, y ≠ ε where the middle piece isn't empty, where the middle piece can be xyiz ∈ L replicated zero or more times. Counting Symbols ● Consider the alphabet Σ = { 0, 1 } and the language L = { w ∈ Σ* | w contains an equal number of 0s and 1s. } ● For example: ● 01 ∈ L ● 110010 ∈ L ● 11011 ∉ L ● Question: Is L a regular language? The Weak Pumping Lemma L = { w ∈ {0, 1}*| w contains an equal number of 0s and 1s. } 1 0 0 1 An Incorrect Proof Theorem: L is regular. Proof: We show that L satisfies the condition of the pumping lemma. Let n = 2 and consider any string w ∈ L such that |w| ≥ 2. Then we can write w = xyz such that x = z = ε and y = w, so y ≠ ε. Then for any natural number i, xyiz = wi, which has the same number of 0s and 1s. Since L passes the conditions of the weak pumping lemma, L is regular. ■ The Weak Pumping Lemma ● The Weak Pumping Lemma for Regular Languages states that ThisThis sayssays nothingnothing aboutabout For any regular language L, languageslanguages thatthat aren'taren't regular!regular! There exists a positive natural number n such that For any w ∈ L with |w| ≥ n, There exists strings x, y, z such that For any natural number i, w = xyz, w can be broken into three pieces, y ≠ ε where the middle piece isn't empty, where the middle piece can be xyiz ∈ L replicated zero or more times. Caution with the Pumping Lemma ● The weak and full pumping lemmas describe a necessary condition of regular languages. ● If L is regular, L passes the conditions of the pumping lemma. ● The weak and full pumping lemmas are not a sufficient condition of regular languages. ● If L is not regular, it still might pass the conditions of the pumping lemma! ● If a language fails the pumping lemma, it is definitely not regular. ● If a language passes the pumping lemma, we learn nothing about whether it is regular or not. L is Not Regular ● The language L can be proven not to be regular using a stronger version of the pumping lemma. ● To see the full pumping lemma, we need to revisit our original insight. An Important Observation 0, 1 1 0, 1 q5 0 q2 1 1 start 0 0 1 q0 q1 q3 q4 0 0 1 1 0 1 1 0 1 1 0 1 1 1 q0 q1 q2 q3 q1 q2 q3 q1 q2 q3 q1 q2 q3 q4 Pumping Lemma Intuition ● Let D be a DFA with n states. ● Any string w accepted by D that has length at least n must visit some state twice within its first n characters. ● Number of states visited is equal n + 1. ● By the pigeonhole principle, some state is duplicated. ● The substring of w in-between those revisited states can be removed, duplicated, tripled, quadrupled, etc. without changing the fact that w is accepted by D. The Pumping Lemma For any regular language L, There exists a positive natural number n such that For any w ∈ L with |w| ≥ n, There exists strings x, y, z such that For any natural number i, w = xyz, w can be broken into three pieces, |xy| ≤ n, where the first two pieces occur at the start of the string, y ≠ ε where the middle piece isn't empty, xyiz ∈ L where the middle piece can be replicated zero or more times. Why This Change Matters ● The restriction |xy| ≤ n means that we can limit where the string to pump must be. ● If we specifically craft the first n characters of the string to pump, we can force y to have a specific property. ● We can then show that y cannot be pumped arbitrarily many times. The Pumping Lemma L = { w ∈ {0, 1}*| w contains an equal number of 0s and 1s. } Suppose the pumping length is 4. 0 0 0 0 1 1 1 1 ≤ SinceSince |xy||xy| ≤ 4,4, thethe stringstring toto pumppump mustmust bebe somewheresomewhere inin here.here. L = { w ∈ {0, 1}* | w has an equal number of 0s and 1s } Theorem: L is not regular. Proof: By contradiction; assume that L is regular. Let n be the length guaranteed by the pumping lemma. Consider the string w = 0n1n. Then |w| = 2n ≥ n and w ∈ L. Therefore, there exist strings x, y, and z such that w = xyz, |xy| ≤ n, y ≠ ε, and for any natural number i, xyiz ∈ L. Since |xy| ≤ n, y must consist solely of 0s. But then xy2z = 0n+|y|1n, and since |y| > 0, we have that xy2z ∉ L. We have reached a contradiction, so our assumption was wrong and L is not regular. ■ Summary of the Pumping Lemma ● Using the pigeonhole principle, we can prove the weak pumping lemma and pumping lemma. ● These lemmas describe essential properties of the regular languages. ● Any language that fails to have these properties cannot be regular. Beyond Finite Automata Where We Are ● Our study of the regular languages gives us an exact characterization of problems that can be solved by finite computers. ● Not all languages are regular. ● How do we build more powerful computing devices? The Problem ● Finite automata accept precisely the regular languages. ● We may need unbounded memory to recognize context-free languages. ● e.g. { 0n1n | n ∈ ℕ } requires unbounded counting. ● How do we build an automaton with finitely many states but unbounded memory? TheThe finite-statefinite-state controlcontrol actsacts asas aa finitefinite memory.memory. TheThe inputinput tapetape holdsholds thethe inputinput string.string. A B C A ... Memory Device WeWe cancan addadd anan infiniteinfinite memorymemory devicedevice thethe finite-statefinite-state controlcontrol cancan useuse toto storestore information.information. Adding Memory to Automata ● We can augment a finite automaton by adding in a memory device for the automaton to store extra information. ● The finite automaton now can base its transition on both the current symbol being read and values stored in memory. ● The finite automaton can issue commands to the memory device whenever it makes a transition. ● e.g. add new data, change existing data, etc. Stack-Based Memory ● There are many types of memory that we might give to an automaton. ● We'll see at least two this quarter. ● One of the simplest types of memory is a stack. b a Stack-Based Memory ● Only the top of the stack is visible at any point in time. ● New symbols may be pushed onto the stack, which cover up the old stack top. ● The top symbol of the stack may be popped, exposing the symbol below it. Pushdown Automata ● A pushdown automaton (PDA) is a finite automaton equipped with a stack-based memory. ● Each transition ● is based on the current input symbol and the top of the stack, ● optionally pops the top of the stack, and ● optionally pushes new symbols onto the stack. ● Initially, the stack holds a special symbol Z0 that indicates the bottom of the stack. Our First PDA ● Consider the language L = { w ∈ Σ* | w is a string of balanced digits } over Σ = { 0, 1 } ● We can exploit the stack to our advantage: ● Whenever we see a 0, push it onto the stack. ● Whenever we see a 1, pop the corresponding 0 from the stack (or fail if not matched) ● When input is consumed, if the stack is empty, accept. A Simple Pushdown Automaton 0, Z0 → 0Z0 0, 0 → 00 1, 0 → ε ε, Z → ε start 0 A Simple Pushdown Automaton ToTo findfind anan applicableapplicable 0, Z0 → 0Z0 transition,transition, matchmatch thethe 0, 0 → 00 current input/stack pair. 1, 0 → ε current input/stack pair. ε, Z → ε start 0 AA transitiontransition ofof thethe formform a,a, b b → → z z Z0 MeansMeans “If“If thethe currentcurrent inputinput symbolsymbol isis aa andand thethe currentcurrent stackstack symbolsymbol isis b,b, thenthen 0 0 0 1 1 1 followfollow thisthis transition,transition, poppop b,b, andand pushpush thethe stringstring z.z. A Simple Pushdown Automaton 0, Z0 → 0Z0 0, 0 → 00 1, 0 → ε ε, Z → ε start 0 IfIf aa transitiontransition readsreads thethe toptop symbolsymbol ofof thethe stack,stack, itit alwaysalways popspops thatthat symbolsymbol (though(though itit mightmight replacereplace it)it) 0 0 0 1 1 1 A Simple Pushdown Automaton 0, Z0 → 0Z0 0, 0 → 00 1, 0 → ε ε, Z → ε start 0 EachEach transitiontransition thenthen pushespushes somesome Z (possibly(possibly empty)empty) stringstring backback ontoonto thethe 0 0 stack.stack. NoticeNotice thatthat thethe leftmostleftmost symbolsymbol isis pushedpushed ontoonto thethe top.top. 0 0 0 1 1 1 A Simple Pushdown Automaton 0, Z0 → 0Z0 0, 0 → 00 1, 0 → ε ε, Z → ε start 0 ε WeWe nownow pushpush thethe stringstring ε ontoonto the stack, which adds no new the stack, which adds no new 0 0 Z0 characters.characters. ThisThis essentiallyessentially meansmeans “pop“pop thethe stack.”stack.” 0 0 0 1 1 1 A Simple Pushdown Automaton 0, Z0 → 0Z0 0, 0 → 00 1, 0 → ε ε, Z → ε start 0 ThisThis transitiontransition cancan bebe takentaken atat anyany time Z0 is atop the stack, but we've Z time Z0 is atop the stack, but we've 0 nondeterministicallynondeterministically guessedguessed thatthat thisthis wouldwould bebe aa goodgood timetime toto taketake it.it.
Load more

Recommended publications
  • More Concise Representation of Regular Languages by Automata and Regular Expressions
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Information and Computation 208 (2010)385–394 Contents lists available at ScienceDirect Information and Computation journal homepage: www.elsevier.com/locate/ic More concise representation of regular languages by automata ୋ and regular expressions Viliam Geffert a, Carlo Mereghetti b,∗, Beatrice Palano b a Department of Computer Science, P. J. Šafárik University, Jesenná 5, 04154 Košice, Slovakia b Dipartimento di Scienze dell’Informazione, Università degli Studi di Milano, via Comelico 39, 20135 Milano, Italy ARTICLE INFO ABSTRACT Article history: We consider two formalisms for representing regular languages: constant height pushdown Received 12 February 2009 automata and straight line programs for regular expressions. We constructively prove that Revised 27 July 2009 their sizes are polynomially related. Comparing them with the sizes of finite state automata Available online 18 January 2010 and regular expressions, we obtain optimal exponential and double exponential gaps, i.e., a more concise representation of regular languages. Keywords: © 2010 Elsevier Inc. All rights reserved. Pushdown automata Regular expressions Straight line programs Descriptional complexity 1. Introduction Several systems for representing regular languages have been presented and studied in the literature. For instance, for the original model of finite state automaton [11], a lot of modifications have been introduced: nondeterminism [11], alternation [4], probabilistic evolution [10], two-way input head motion [12], etc. Other important formalisms for defining regular languages are, e.g., regular grammars [7] and regular expressions [8]. All these models have been proved to share the same expressive power by exhibiting simulation results.
    [Show full text]
  • Iterated Stack Automata and Complexity Classes
    INFORMATION AND COMPUTATION 95, 2 1-75 ( t 99 1) Iterated Stack Automata and Complexity Classes JOOST ENCELFRIET Department of Computer Science, Leiden University. P.O. Box 9.512, 2300 RA Leiden, The Netherlands An iterated pushdown is a pushdown of pushdowns of . of pushdowns. An iterated exponential function is 2 to the 2 to the to the 2 to some polynomial. The main result presented here is that the nondeterministic 2-way and multi-head iterated pushdown automata characterize the deterministic iterated exponential time complexity classes. This is proved by investigating both nondeterministic and alternating auxiliary iterated pushdown automata, for which similar characteriza- tion results are given. In particular it is shown that alternation corresponds to one more iteration of pushdowns. These results are applied to the l-way iterated pushdown automata: (1) they form a proper hierarchy with respect to the number of iterations, and (2) their emptiness problem is complete in deterministic iterated exponential time. Similar results are given for iterated stack (checking stack, non- erasing stack, nested stack, checking stack-pushdown) automata. ? 1991 Academic Press. Inc INTRODUCTION It is well known that several types of 2-way and multi-head pushdown automata and stack automata have the same power as certain time or space bounded Turing machines; see, e.g., Chapter 14 of (Hopcroft and Ullman, 1979), or Sections 13 and 20.2 of (Wagner and Wechsung, 1986). For the deterministic and nondeterministic case such characterizations were given by Fischer (1969) for checking stack automata, by Hopcroft and Ullman (1967b) for nonerasing stack automata, by Cook (1971) for auxiliary pushdown and 2-way stack automata, by Ibarra (1971) for auxiliary (nonerasing and erasing) stack automata, by Beeri (1975) for 2-way and auxiliary nested stack automata, and by van Leeuwen (1976) for auxiliary checking stack-pushdown automata.
    [Show full text]
  • 2 Context-Free Grammars and Pushdown Automata
    2 Context-free Grammars and Pushdown Automata As we have seen in Exercise 15, regular languages are not even sufficient to cover well-balanced brackets. In order even to treat the problem of parsing a program we have to consider more powerful languages. Our aim in this section is to generalize the concept of a regular language to cover this example (and others like it), and to find ways of describing such languages. The analogue of a regular expression is a context-free grammar while finite state machines are generalized to pushdown automata. We establish the correspondence between the two and then finish by describing the kinds of languages that are not captured by these more general methods. 2.1 Context-free grammars The question of recognizing well-balanced brackets is the same as that of dealing with nesting to arbitrary depths, which certainly occurs in all programming languages. To cover these examples we introduce the following definition. Definition 11 A context-free grammar is given by the following: • an alphabet Σ of terminal symbols, also called the object alphabet; • an alphabet N of non-terminal symbols, the elements of which are also referred to as auxiliary characters, placeholders or, in some books, variables, where N \ Σ = ;;7 • a special non-terminal symbol S 2 N called the start symbol; • a finite set of production rules, that is strings of the form R ! Γ where R 2 N is a non- terminal symbol and Γ 2 (Σ [ N)∗ is an arbitrary string of terminal and non-terminal symbols, which can be read as `R can be replaced by Γ'.8 You will meet grammars in more detail in the other part of the course, when examining parsers.
    [Show full text]
  • Input Finite State Control Accept Or Reject Stack
    Pushdown Automata CS351 Just as a DFA was equivalent to a regular expression, we have a similar analogy for the context-free grammar. A pushdown automata (PDA) is equivalent in power to context- free grammars. We can give either a context-free grammar generating a context-free language, or we can give a pushdown automaton. Certain languages are more easily described in forms of one or the other. A pushdown automata is quite similar to a nondeterministic finite automata, but we add the extra component of a stack. The stack is of infinite size, and this is what will allow us to recognize some of the non-regular languages. We can visualize the pushdown automata as the following schematic: Finite input Accept or reject State Control Stack The finite state control reads inputs, one symbol at a time. The pushdown automata makes transitions to new states based on the input symbol, current state, and the top of the stack. In the process we may manipulate the stack. Ultimately, the control accepts or rejects the input. In one transition the PDA may do the following: 1. Consume the input symbol. If ε is the input symbol, then no input is consumed. 2. Go to a new state, which may be the same as the previous state. 3. Replace the symbol at the top of the stack by any string. a. If this string is ε then this is a pop of the stack b. The string might be the same as the current stack top (does nothing) c. Replace with a new string (pop and push) d.
    [Show full text]
  • Context Free Grammars October 16, 2019 14.1 Review 14.2 Pushdown
    CS 4510: Automata and Complexity Fall 2019 Lecture 14: Context Free Grammars October 16, 2019 Lecturer: Santosh Vempala Scribe: Aditi Laddha 14.1 Review Context-free grammar: Context-free grammar(CFG), we define the following: V : set of variables, S 2 V : start symbol, Σ: set of terminals, and R: production rules, V ! (V [ Σ)∗ 14.2 Pushdown Automaton Pushdown Automaton: A pushdown automaton(PDA) is defined as Q: the set of states, q0 2 Q: the start state, F ⊆ Q: the set of accept states, Σ: Input alphabet, Γ: Stack alphabet, and δ: the transition function for PDA where δ : Q × (Σ [ ) × (Γ [ ) ! 2Q×(Γ[). Note that the transition function is non-deterministic and defines the following stack operations: • Push: (q; a; ) ! (q0; b0) In state q on reading input letter a, we go to state q0 and push b0 on top of the stack. • Pop: (q; a; b) ! (q0; ) In state q on reading input letter a and top symbol of the stack b, we go to state q0 and pop b from top of the stack. Example 1: L = f0i1i : i 2 Ng∗ • CFG G1: S ! S ! SS1 S1 ! S1 ! 0S11 • PDA: 14-1 Lecture 14: Context Free Grammars 14-2 0; ! 0 1; 0 ! , ! $ 1; 0 ! start qstart q1 q2 , $ ! qaccept Figure 14.1: PDA for L Example 2: The set of balanced parenthesis • CFG G2: S ! S ! SS S ! (S) • PDA: (; ! ( , ! $ start qstart q1 , $ ! ); (! qaccept Figure 14.2: PDA for balanced parenthesis How can we generate (()(())) using these production rules? There can be multiple ways to generate the same terminal string from S.
    [Show full text]
  • Using Jflap in Academics to Understand Automata Theory in a Better Way
    International Journal of Advances in Electronics and Computer Science, ISSN: 2393-2835 Volume-5, Issue-5, May.-2018 http://iraj.in USING JFLAP IN ACADEMICS TO UNDERSTAND AUTOMATA THEORY IN A BETTER WAY KAVITA TEWANI Assistant Professor, Institute of Technology, Nirma University E-mail: [email protected] Abstract - As it is been observed that the things get much easier once we visualize it and hence this paper is just a step towards it. Usually many subjects in computer engineering curriculum like algorithms, data structures, and theory of computation are taught theoretically however it lacks the visualization that may help students to understand the concepts in a better way. The paper shows some of the practices on tool called JFLAP (Java Formal Languages and Automata Package) and the statistics on how it affected the students to learn the concepts easily. The diagrammatic representation is used to help the theory mentioned. Keywords - Automata, JFLAP, Grammar, Chomsky Heirarchy , Turing Machine, DFA, NFA I. INTRODUCTION visualization and interaction in the course is enhanced through the popular software tools[6,7]. The proper The theory of computation is a basic course in use of JFLAP and instructions are provided in the computer engineering discipline. It is the subject manual “JFLAP activities for Formal Languages and which is been taught theoretically and hands-on Automata” by Peter Linz and Susan Rodger. It problems are given to the students to enhance the includes the theoretical approach and their understanding but it lacks the programming implementation in JFLAP. [10] assignment. However the subject is the base for creation of compilers and therefore should be given III.
    [Show full text]
  • Visibly Pushdown Transducers
    Facult´edes Sciences Appliqu´ees Universit´eLibre de Bruxelles D´epartement CoDE Visibly Pushdown Transducers Fr´ed´eric SERVAIS Th`ese pr´esent´ee en vue de l’obtention du grade de Docteur en Sciences Appliqu´ees Sous la direction du Prof. Esteban Zim´anyi et co-direction du Prof. Jean-Fran¸cois Raskin Visibly Pushdown Transducers Fr´ed´eric SERVAIS Th`ese r´ealis´ee sous la direction du Professeur Esteban Zim´anyi et co-direction du Professeur Jean-Fran¸cois Raskin et pr´esent´ee en vue de l’obtention du grade de Doc- teur en Sciences Appliqu´ees. La d´efense publique a eu lieu le 26 septembre 2011 `a l’Universit´eLibre de Bruxelles, devant le jury compos´ede: • Rajeev Alur (University of Pennsylvania, United States) • Emmanuel Filiot (Universit´eLibre de Bruxelles, Belgium) • Sebastian Maneth (National ICT Australia (NICTA)) • Frank Neven (University of Hasselt, Belgium) • Jean-Fran¸cois Raskin (Universit´eLibre de Bruxelles, Belgium) • Stijn Vansummeren (Universit´eLibre de Bruxelles, Belgium) • Esteban Zim´anyi (Universit´eLibre de Bruxelles, Belgium) A` mes parents iii R´esum´e Dans ce travail nous introduisons un nouveau mod`ele de transducteurs, les trans- ducteurs avec pile `acomportement visible (VPT), particuli`erement adapt´eau traite- ment des documents comportant une structure imbriqu´ee. Les VPT sont obtenus par une restriction syntaxique de transducteurs `apile et sont une g´en´eralisation des transducteurs finis. Contrairement aux transducteurs `a piles, les VPT jouissent de relativement bonnes propri´et´es. Nous montrons qu’il est d´ecidable si un VPT est fonctionnel et plus g´en´eralement s’il d´efinit une relation k-valu´ee.
    [Show full text]
  • Using JFLAP to Interact with Theorems in Automata Theory
    Usi ng JFL AP to Interact wi th Theorems in Automata Theory Eric Gra mond and Susan H. Ro dger Duke Uni versity, Durham, NC ro dger@cs. duke. edu ract Abst son is that students do not receive immediate feedback when working problems using pencil and paper. Weaker students An automata theory course can be taught in an interactive, especially need to work additional problems to understand hands-on manner using a computer. At Duke we have been concepts, and need to know if their solutions are correct. using the software tool JFLAP to provide interaction and The notation in the automata theory course is needed to feedback in CPS 140, our automata theory course. JFLAP describe theoretical representations of languages (automata is a tool for designing and running nondeterministic ver- and grammars). Tools such as JFLAP provide students an sions of finite automata, pushdown automata, and Turing alternative visual representation and a chance to create and machines. Recently, we have enhanced JFLAP to allow one simulate these representations. As students interact and re- to study the proofs of several theorems that focus on conver- ceive feedback on concepts with the visual representations sions of languages, from one form to another, such as con- provided in JFLAP, they may become more comfortable with verting an NFA to a DFA and then to a minimum state DFA. the mathematical notation used in the formal representation. In addition, our enhancements combined with other tools al- Recently we have extended JFLAP to allow one to in- low one to interactively study LL and LR parsing methods.
    [Show full text]
  • Pushdown Automata 1 Definition 2 Example 3 Intuition Behind The
    Pushdown Automata 1 Definition A Pushdown Automaton(PDA) is similar to a non deterministic finite automaton with the addition of a stack. A stack is simply a last in first out structure, easily visualized as a Pez Dispenser or a stack of plates. The stacks in pushdown automata are allowed to grow indefinitely and as a result of that, these machines can recognize languages that cannot be recognized by NFAs. It is also provable that the set of languages defined by PDA are exactly the context free languages. Formally, a pushdown automata is defined as a 6-tuple (Q; Σ; Γ; δ; q0;F ) where • Q is a finite set of states • Σ is the alphabet of the language. Also a finite set. • Γ is the stack alphabet. It is also a finite set and is allowed to have elements that are not in Σ. In particular, in JFLAP, the initial state of the stack is assumed to be the start symbol Z. • δ is the transition function. The transition function is the critical piece in designing a PDA. The function takes 3 arguments - a state q, an input alphabet σ and a character γ that is on top of the stack. What is returned is the state that you go to as well as the symbol that is put on top of the stack in place of γ. • q0 as usual is used to denote the start state • F ⊆ Q is a set of final states 2 Example We will now learn how to use JFLAP to build a PDA that recognizes the following language n 2n L1 = f0 1 jn > 0g.
    [Show full text]
  • Regular Languages and Finite Automata for Part IA of the Computer Science Tripos
    N Lecture Notes on Regular Languages and Finite Automata for Part IA of the Computer Science Tripos Prof. Andrew M. Pitts Cambridge University Computer Laboratory c 2013 A. M. Pitts Contents Learning Guide ii 1 Regular Expressions 1 1.1 Alphabets,strings,andlanguages . .......... 1 1.2 Patternmatching................................. .... 4 1.3 Somequestionsaboutlanguages . ....... 6 1.4 Exercises....................................... .. 8 2 Finite State Machines 11 2.1 Finiteautomata .................................. ... 11 2.2 Determinism, non-determinism, and ε-transitions. .. .. .. .. .. .. .. .. .. 14 2.3 Asubsetconstruction . .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..... 17 2.4 Summary ........................................ 20 2.5 Exercises....................................... .. 20 3 Regular Languages, I 23 3.1 Finiteautomatafromregularexpressions . ............ 23 3.2 Decidabilityofmatching . ...... 28 3.3 Exercises....................................... .. 30 4 Regular Languages, II 31 4.1 Regularexpressionsfromfiniteautomata . ........... 31 4.2 Anexample ....................................... 32 4.3 Complementand intersectionof regularlanguages . .............. 34 4.4 Exercises....................................... .. 36 5 The Pumping Lemma 39 5.1 ProvingthePumpingLemma . .... 40 5.2 UsingthePumpingLemma . ... 41 5.3 Decidabilityoflanguageequivalence . ........... 44 5.4 Exercises....................................... .. 45 6 Grammars 47 6.1 Context-freegrammars . ..... 47 6.2 Backus-NaurForm ................................
    [Show full text]
  • CPS 220 – Theory of Computation
    CPS 220 – Theory of Computation Pushdown Automata (PDA) Nondeterministic Finite Automaton with some extra memory Memory is called the stack, accessed in a very restricted way: in a First-In First-Out fashion (FIFO) FIFO That is, elements can be “pushed” in the memory. At any moment, only the last element pushed in memory can be accessed. To see elements below that, the top element has to be “popped” out of the stack and lost. Still this memory results in the PDA having much more power as a computational model, than the NFA without the memory. NFA Required for the PDA to be able to recognize the same languages as the CFGs. Figure of a PDA: State Control Input Stack . Example: A = { 0n 1n | n ≥ 0 } -not regular - not recognized by any FA PDA construction: Push 0s in the stack. Then, once 1s start to be read, pop 0s to match the 1s. If the number of 1s is exactly the same as the number of 0s pushed, accept. In all other situations, reject. Formal Definition of Pushdown Automata A Pushdown Automaton (PDA) is an 8-tuple P = (Q, Σ, Γ, δ, q0, Z0, F), where: 1. Q is a finite set of states. 2. Σ is a finite set of input symbols. 3. Γ is a finite set of symbols that can be pushed on the stack. (Usually all of Σ plus a few special symbols.) Q × Γε 4. δ : Q × Σε × Γε → 2 is the transition function. δ (state, sybmol, stack symbol)→(new state, new stack symbol to push) 5.
    [Show full text]
  • Finite-State Machines and Pushdown Automata
    CHAPTER Finite-State Machines and Pushdown Automata The finite-state machine (FSM) and the pushdown automaton (PDA) enjoy a special place in computer science. The FSM has proven to be a very useful model for many practical tasks and deserves to be among the tools of every practicing computer scientist. Many simple tasks, such as interpreting the commands typed into a keyboard or running a calculator, can be modeled by finite-state machines. The PDA is a model to which one appeals when writing compilers because it captures the essential architectural features needed to parse context-free languages, languages whose structure most closely resembles that of many programming languages. In this chapter we examine the language recognition capability of FSMs and PDAs. We show that FSMs recognize exactly the regular languages, languages defined by regular expres- sions and generated by regular grammars. We also provide an algorithm to find a FSM that is equivalent to a given FSM but has the fewest states. We examine language recognition by PDAs and show that PDAs recognize exactly the context-free languages, languages whose grammars satisfy less stringent requirements than reg- ular grammars. Both regular and context-free grammar types are special cases of the phrase- structure grammars that are shown in Chapter 5 to be the languages accepted by Turing ma- chines. It is desirable not only to classify languages by the architecture of machines that recog- nize them but also to have tests to show that a language is not of a particular type. For this reason we establish so-called pumping lemmas whose purpose is to show how strings in one language can be elongated or “pumped up.” Pumping up may reveal that a language does not fall into a presumed language category.
    [Show full text]